This invention relates to methods of making nonlinear optical structures from suitable polymers, and the resulting structures.
Nonlinear optical (NLO) polymers attract a great deal of attention due to their potential applications in photonic technologies. NLO materials exhibit an induced polarization that is nonlinear with an incident electromagnetic (EM) field, even at modest incident EM intensities. For a material to exhibit even-order NLO effects, such as second harmonic generation (SHG), the material must be non-centrosymmetric. Therefore, macroscopic even-order NLO effects are present in NLO polymers only if their NLO chromophores are assembled in an acentric arrangement.
The conventional method for aligning chromophores in polymer thin films is to use an external strong electric field to pole the chromophores along a prescribed direction. See, for example, M. Mortazavi et al., J. Opt. Soc. Am. B., 6:733 (1989). However, since the thin films are ultimately removed from the strong field, the chromophores exist in a thermodynamically unstable state and tend to randomize over time, thereby degrading the NLO properties over time.
Other methods involve processes in which the chromophores self-assemble into a noncentrosymmetric alignment, as opposed to forced assembly. One such method involves the formation of acentric Y-type Langmuir Blodgett (LB) films incorporating NLO chromophores. See, for example, G. J. Ashwell et al., Nature, 357:393 (1992). Another method involves the formation of oriented multilayers using tailored chemisorption processes. See, for example, D. Li et al., J. Am. Chem. Soc., 112:7389 (1990).